Purpose:
Demonstration of an instrument that measures the dynamic properties of the topography of the human tear film to sub-micron accuracies.

Methods:
The Tear Film Interferometer (TFI) is an instantaneous phase-shifting Twyman-Green interferometer. The system measures the high-spatial frequency variations of the surface topography of the in vivo tear film at video rates. The laser source is a near-infrared solid-state laser (785 nm) that has been carefully designed and calibrated to ensure that the system operates at eye safe levels. As currently configured, measurements are made over a 6 mm diameter area on the cornea. A fixation target is used to minimize eye motion. Four simultaneous phase-shifted interferometric images are captured simultaneously which enables a stable eye position and hydration level during the acquisition. Successive frames of interferometric height measurements are combined to produce movies showing both the quantitative and qualitative changes in the topography of the tear film surface and structure for periods of up to 20 seconds at 29 frames per second.

Results:
To date, seven human subjects have been examined using the TFI. The TFI has been demonstrated to have a surface height resolution of ±25 nm and spatial resolution of 6 μm. Examples of features that have been observed in these in preliminary studies in the tear film include: post-blink disruption, evolution, and stabilization of the tear film; tear film artifacts generated by blinking; tear film evaporation and break-up; and the propagation of foreign objects in the tear film.

Conclusions:
The capabilities of the Tear Film Interferometer have been demonstrated as a viable technology for characterizing the dynamic topography of the corneal tear film with high resolution. The combination of height resolution, spatial resolution and measurement area associated with this instrument should enable advances in the basic science of the human tear film.